A variety of electrochemical cells falls within a category of cells often referred to as solid polymer electrolyte (“SPE”) cells. An SPE cell typically employs a membrane of a cation exchange polymer that serves as a physical separator between the anode and cathode while also serving as an electrolyte. SPE cells can be operated as electrolytic cells for the production of electrochemical products or they may be operated as fuel cells.
Fuel cells are electrochemical cells that convert reactants, namely fuel and oxidant fluid streams, to generate electric power and reaction products. A broad range of reactants can be used in fuel cells and such reactants may be delivered in gaseous or liquid streams. For example, the fuel stream may be substantially pure hydrogen gas, a gaseous hydrogen-containing reformate stream, or an aqueous alcohol, for example methanol in a direct methanol fuel cell (DMFC). The oxidant may, for example, be substantially pure oxygen or a dilute oxygen stream such as air.
In SPE fuel cells, the solid polymer electrolyte membrane is typically perfluorinated sulfonic acid polymer membrane in acid form. Such fuel cells are often referred to as proton exchange membrane (“PEM”) fuel cells. The membrane is disposed between and in contact with the anode and the cathode. Electrocatalysts in the anode and the cathode typically induce the desired electrochemical reactions and may be, for example, a metal black, an alloy or a metal catalyst supported on a substrate, e.g., platinum on carbon. SPE fuel cells typically also comprise a porous, electrically conductive sheet material that is in electrical contact with each of the electrodes, and permit diffusion of the reactants to the electrodes. In fuel cells that employ gaseous reactants, this porous, conductive sheet material is sometimes referred to as a gas diffusion layer and is suitably provided by a carbon fiber paper or carbon cloth. An assembly including the membrane, anode and cathode, and gas diffusion layers for each electrode, is sometimes referred to as a membrane electrode assembly (“MEA”). Bipolar plates, made of a conductive material and providing flow fields for the reactants, are placed between a number of adjacent MEAs. A number of MEAs and bipolar plates are assembled in this manner to provide a fuel cell stack.
For the electrodes to function effectively in SPE fuel cells, effective electrocatalyst sites must be provided. Effective electrocatalyst sites have several desirable characteristics: (1) the sites are accessible to the reactant, (2) the sites are electrically connected to the gas diffusion layer, and (3) the sites are ionically connected to the fuel cell electrolyte. In order to improve ionic conductivity, ion exchange polymers are often incorporated into the electrodes. In addition, incorporation of ion exchange polymer into the electrodes can also have beneficial effects with liquid feed fuels. For example, in a direct methanol fuel cell, ion exchange polymer in the anode makes it more wettable by the liquid feed stream in order to improve access of the reactant to the electrocatalyst sites.
In electrodes for some fuel cells employing gaseous feed fuels, hydrophobic components such as polytetrafluoroethylene (“PTFE”) are typically employed, in part, to render electrodes less wettable and to prevent “flooding”. Flooding generally refers to a situation where the pores in an electrode become filled with water formed as a reaction product, such that the flow of the gaseous reactant through the electrode becomes impeded.
Essentially two approaches have been taken to form electrodes for SPE fuel cells. In one, the electrodes are formed on the gas diffusion layers by coating electrocatalyst and dispersed particles of PTFE in a suitable liquid medium onto the gas diffusion layer, e.g., carbon fiber paper. The carbon fiber paper with the electrodes attached and a membrane are then assembled into an MEA by pressing such that the electrodes are in contact with the membrane. In MEA's of this type, it is difficult to establish the desired ionic contact between the electrode and the membrane due to the lack of intimate contact. As a result, the interfacial resistance may be higher than desired. In the other main approach for forming electrodes, electrodes are formed onto the surface of the membrane. A membrane having electrodes so formed is often referred to as a catalyst coated membrane (“CCM”). Employing CCMs can provide improved performance over forming electrodes on the gas diffusion layer but CCMs are typically more difficult to manufacture.
Various manufacturing methods have been developed for manufacturing CCMs. Many of these processes have employed electrocatalyst coating slurries containing the electrocatalyst and the ion exchange polymer and, optionally, other materials such as a PTFE dispersion. The ion exchange polymer in the membrane itself, and in the electrocatalyst coating solution could be employed in either hydrolyzed or unhydrolyzed ion-exchange polymer (sulfonyl fluoride form when perfluorinated sulfonic acid polymer is used), and in the latter case, the polymer must be hydrolyzed during the manufacturing process. Techniques that use unhydrolyzed polymer in the membrane, electrocatalyst composition or both can produce excellent CCMs but are difficult to apply to commercial manufacture because a hydrolysis step and subsequent washing steps are required after application of the electrode. In some techniques, a “decal” is first made by depositing the electrocatalyst coating solution on another substrate, removing the solvent and then transferring and adhering the resulting decal to the membrane. These techniques also can produce good results but mechanical handling and placement of decals on the membrane are difficult to perform in high volume manufacturing operations.
A variety of techniques have been developed for CCM manufacture which apply an electrocatalyst coating solution containing the ion exchange polymer in hydrolyzed form directly to membrane also in hydrolyzed form. However, the known methods again are difficult to employ in high volume manufacturing operations. Known coating techniques such as spraying, painting, patch coating and screen printing are typically slow, can cause loss of valuable catalyst and require the application of relatively thick coatings. Thick coatings contain a large amount of solvent and cause swelling of the membrane that causes it to sag, slump, or droop, resulting in loss of dimensional control of the membrane, handling difficulties during processing, and poor electrode formation. Attempts have been made to overcome such problems for mass production processes. For example, in U.S. Pat. No. 6,074,692, a slurry containing the electrocatalyst in a liquid vehicle such as ethylene or propylene glycol is sprayed on the membrane while the membrane is held in a tractor clamp feed device. This patent teaches pretreating the membrane with the liquid vehicle prior to the spraying operation to decrease the swelling problems. However, processes employing such pretreatment steps are complicated, difficult to control, and require the removal of large amounts of the vehicle in a drying operation. Such drying operations are typically slow and require either disposal or recycling of large quantities of the vehicle to comply with applicable environmental requirements.
Accordingly, a process is needed which is suitable for the high volume production of catalyst coated membranes and which avoids problems associated with prior art processes. Further, a process is needed which is suitable for the direct application of an electrocatalyst coating composition to a membrane in hydrolyzed form which avoids the swelling problems associated with known processes and which does not require complicated pre-treatment or post-treatment process steps.